Abstract: By comparing the differences between the Roothaan-Hartree-Fock function, configuration interaction wave function and Hylleraas-type wave function of Helium ground state, we present intuitive figures to show the reason why Hylleraas-type wave function is more accurate than the others is that it reflects the Coulomb correlation between electrons correctly. A careful analysis of Hylleraas function indicates that Coulomb correlation tends to make the two electrons on the opposite side of nucleus; a simple model of Helium is proposed accordingly which can produce the ground state energy agreement with the experiment.

Key words: Helium, Hylleraas-type wave function, Coulomb correlation, Coulomb hole